The major chemical components of cells include the nucleic
acids RNA and DNA, polysaccharides (carbohydrates),
fatty materials (lipids), and many thousands of different
proteins. Proteins catalyze most of the metabolism,
the network of chemical reactions by which cells construct
their own substance and by which they obtain and utilize
energy for all life processes. Proteins, whose structure is
dictated by the DNA of the corresponding genes, are preciselyconstructed.
As submicroscopicmachines theyhave
moving parts and apparatus for recognizing and binding to
other molecules, both large and small. Some coenzymes
cooperate with the proteins by carrying electrons, atoms,
or molecular fragments. Others help the proteins to catalyze
reactions that are difficult or impossible for the reactive
groups provided by the amino acid side chains of
the proteins.

The B-vitamin-containing coenzymes provide a logical
starting point for our discussion. As mentioned in
Section I, thiamin, nicotinamide, and riboflavin were recognized
early in the 20th century as participants in energy
metabolism in both animal and yeast cells. Panthothenic
acid, biotin, and vitamin B12 were soon added to this list.
We now know, in part from complete genome sequences,
that all living creatures depend upon these coenzymes to
help catalyze a series of central pathways of metabolism.
One of these pathways is utilized by aerobic organisms,
from bacteria to human beings, for the oxidation of fatty
acids (Fig. 12).